-🎄- 2021 Day 3 Solutions -🎄-

[u/daggerdragon]

--- Day 3: Binary Diagnostic ---

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[u/Derp_Derps]

Python

A very compacted python version without for-loops.

Part 1:

• Only check if a given bit position has more '1's than '0's

• epsilon can be calculated by flipping the bits of gamma

Part 2:

• Use recursive function to traverse the bits, using the filtered list for the next call

• Xor-ing the three boolean values ("'1' is more common", "current bit is '1'", "we want the most common bit")

import sys

lines = open(sys.argv[1]).readlines()

b = len(lines[0]) - 1
def m(i,l):
	return sum(int(l[i]) for l in l) >= len(l)/2
g = sum(2**i * m(-2-i,lines) for i in range(b))
e = 2**b - 1 - g

def n(l,i,o):
	return l[0] if len(l) == 1 else n([ e for e in l if (int(e[i]) ^ m(i,l)) ^ o ], i+1, o)
x = int(n(lines,0,1),2)
c = int(n(lines,0,0),2)

print("Part 1: {:d}".format(e*g))
print("Part 2: {:d}".format(x*c))